Research On Wind Farm Relay Protection Considering Frequency Offset

Wind power is mainly connected to the power grid in the form of grid connection,and the protection configuration of the wind farm is still the same as that of the traditional power grid.The unique transient characteristics of the wind farm have not been taken into account,resulting in the wind farm can not been protected reliablely and effectively.Therefore,it is necessary to do further research on the relay protection of the wind farm.First,build a doubly-fed wind farm electromagnetic transient model in PSCAD/EMTDC simulation software,and verify the correctness of the model.The simulation found that when the system runs in an asynchronous state before the system failure,the frequency of the short-circuit current on the wind farm side offset.Therefore,the protection based on Fourier power frequency algorithm is no longer applicable.Then,Study on the differential protection of the three-winding transformer in the wind farm.Transformer protection module is built in PSCAD/EMTDC simulation software.Simulation research is performed on the adaptability of the protection under different working conditions.It is found that the differential current and braking current fluctuate due to the frequency shift of the fault current,and part of the action curve falls into the braking zone.In response to this phenomenon,in the case of a light frequency offset,the action boundary is appropriately reduced to ensure the correct action of the protection,but when the frequency offset is serious,this method is no longer feasible.Based on the analysis of the short-circuit current transient characteristics,the Hilbert Huang Transform algorithm is used to obtain the instantaneous frequency of the current on each side of the three-winding transformer after the fault.The protection criterion is proposed according to the difference in the instantaneous frequency of the current on each side.Verify the feasibility of the proposed protection criterion.Finally,the protection of the transmission line of a hybrid wind farm consisting of direct-drive wind farms and doubly-fed wind farms is studied.For longitudinal protection based on cosine similarity,Further simulation analysis on the adaptability of hybrid wind farms under different conditions.The simulation analysis shows that the protection based on the cosine similarity can cope with the various operating conditions,correctly distinguish the faulty phase from the non-faulty phase,providing a solution for the incorrect action of protection.